In recent years, due to advancement of drive circuit systems of motors, it has become possible to perform frequency-control of drive power supply, and as a result, a high speed motor driven by adjustable speed control or driven at a higher frequency than a power frequency has been increasingly in demand. In the high speed motors driven as described above, it is necessary to use rotors having strengths capable of withstanding high speed rotation.
That is, a centrifugal force applied to a rotor is proportional to the rotating-radius and is increased in proportional to the square of a rotational speed. Hence, in medium-sized and large-sized high speed motors, a stress more than 600 MPa may be applied to rotors thereof in some cases. Accordingly, for the high speed motors as described above, increase in strengths of the rotor must be achieved.
In addition, in view of recent improvement in motor efficiency, a magnet-embedded type (IPM: Interior Permanent Magnet) DC inverter control motor, in which permanent magnets are embedded in a rotor, has also been increasingly in demand. In the motor described above, magnets embedded in the rotor are liable to jump out therefrom, and in order to prevent the magnets from jumping out, a large force is applied to an electrical steel sheet used for the rotor. From this point of view, an electrical steel sheet for use in the motor, in particular, for use in the rotor has been required to have high strengths.
Since rotating devices such as motors and generators exploit electromagnetic phenomena, core materials therefor are required to have magnetic properties. In particular, the core materials preferably have a low iron loss and a high magnetic flux density.
In general, for assembling an iron core of a rotor, non-oriented electrical steel sheets are formed by punching using a press machine and are then laminated to each other for the use. However, when a core material of rotors used for high speed motors cannot satisfy the mechanical strengths described above, instead of that, a rotor made of cast steel having higher strengths must be used. However, since the cast steel-made rotor mentioned above is a bulk product, compared to a rotor formed of electrical steel sheets laminated to each other, a ripple loss affecting the rotor is large, thereby primarily causing decrease in motor efficiency. The ripple loss indicates an eddy current loss caused by a high frequency magnetic flux.
Accordingly, an electrical steel sheet having superior magnetic properties and high strengths has been desired as a core material for rotors.
As a strengthening method from a metallurgical point of view, for example, solid solution strengthening, precipitation strengthening, and grain-refining strengthening have been known, and there are examples in which some methods mentioned above were applied to electrical steel sheets. For example, according to Japanese Unexamined Patent Application Publication No. 60-238421, based on the results of investigation on advantages and disadvantages of the each strengthening method mentioned above, as a method having the least influence on magnetic properties, the use of solid solution strengthening has been proposed. In addition, a method has been disclosed in which, besides increase of the content of Si to 3.5% to 7.0% (mass percent, hereinafter, the same as above), an element having high capability of solid solution strengthening is added.
In addition, in Japanese Unexamined Patent Application Publication No. 62-256917, a method for controlling the diameter of recrystallized grains has been disclosed in which the content of Si is set in the range of from 2.0% to 3.5%, the content of Ni or the contents of Ni and Mo are increased, and low-temperature annealing at a temperature of 650 to 850° C. is performed. Furthermore, as a method using precipitation strengthening, in Japanese Unexamined Patent Application Publication No. 6-330255, a method has been disclosed in which the content of Si is set in the range of from 2.0% to 4.0% and fine carbides and nitrides of Nb, Zr, Ti, and/or V are precipitated.
By the methods described above, electrical steel sheets can be obtained having a high strength to a certain extent. However, when steel is used in which the contents of Si and an element for solid solution strengthening are high, as disclosed in Japanese Unexamined Patent Application Publication No. 60-238421, cold rolling properties are extremely degraded, and as a result, it becomes disadvantageously difficult to perform stable industrial manufacturing. Furthermore, a problem may arise in that magnetic flux density B50 of the steel sheet obtained by this technique is also seriously decreased to 1.56 to 1.60 T.
In the method disclosed in Japanese Unexamined Patent Application Publication No. 62-256917, in order to increase the mechanical strengths, the growth of recrystallized grains must be suppressed by low-temperature annealing, and as a result, in a relatively low frequency range, for example, of from a power frequency (approximately 50 Hz) to several hundred Hertz, a problem occurs in that the iron loss is decreased.
Accordingly, the electrical steel sheet obtained by the method disclosed in Japanese Unexamined Patent Application Publication No. 62-256917 cannot be used as a material for a stator member since the iron loss of this application is important in this frequency range. Hence, an extreme decrease in yield of the electrical steel sheet according to this method could not been avoided. That is, when stator and rotor members are obtained by punching, a ring-shaped stator member is generally punched out from one steel sheet, and from a remaining central part of the same steel sheet, a rotor member is also obtained by punching, thereby reducing waste. However, in the method disclosed in Japanese Unexamined Patent Application Publication No. 62-256917, two types of members must be obtained from different steel sheets by punching, and as a result, the yield is unfavorably decreased.
On the other hand, according to the method disclosed in Japanese Unexamined Patent Application Publication No. 6-330255, since the carbides and nitrides themselves function as a barrier to magnetic wall movement and interfere with the growth of crystal grains of an electrical steel sheet, the degradation in iron loss is still disadvantageously large.
In addition, regardless of whether any of the methods described above is used, the electrical steel sheets manufactured thereby each have a high hardness, and as a result, the punchabilities thereof are inferior. That is, when the steel sheet for laminated core is punched out, die wear becomes very large, and hence large burrs are liable to be generated in an early stage.
As will be described later, as one of the features of the present invention, the composition of a steel sheet according to the present invention contains a predetermined amount of Cu. Hence, apart from the problems described above, the current status of Cu used in non-oriented electrical steel sheets will be described.
As an example in which Cu is added to an electrical steel sheet, a technique for improving punchabilities has been disclosed in Japanese Unexamined Patent Application Publication No. 62-89816 in which 0.1 to 1.0% of C is added to a steel sheet so as to precipitate graphite. As a method of recrystallization annealing (finish annealing), box annealing is recommended. In this technique, as an element facilitating the precipitation of graphite, Cu in an amount of 1.0% or less is recommended to be added; however, disadvantage in cost is also implied.
However, the electrical steel sheet described above having a composition containing 0.1% or more of C is an exceptional one, and in a general electrical steel sheet, the addition of Cu is not recommended in view of the magnetic properties and the like. For example, in Japanese Unexamined Patent Application Publication No. 9-67654, a non-oriented electrical steel sheet containing more than 1% to 3.5% of Si or the like has been disclosed; however, since the precipitation of CuS and the like has adverse influences on the magnetic properties, the content of Cu is limited to 0.05% or less.
In addition, as a technique which contain a larger amount of Cu than that described above, a method has been disclosed in Japanese Unexamined Patent Application Publication No. 8-295936 in which a non-oriented electrical steel sheet is manufactured from raw materials including scrap steel. In this technique, in order to reduce adverse influences on the magnetic properties caused by alloying elements (0.015% to 0.2% of Cu: 0.01% to 0.5% of Ni: 0.02% to 0.2% of Cr: 0.003% to 0.2% of Sn: and the like) contained in scrap, for example, measures are proposed in which the contents of V and Nb are limited, and in which the diameter of crystal grains after hot-rolled sheet annealing is controlled to 50 μm or less. However, also for this technique, the above elements such as Cu are naturally disadvantageous, and a primary object of this technique is only to reduce the adverse influences of the above elements. In addition, the contents of Cu and the like thus disclosed are small.
Furthermore, as steel which does not contain Si, high-strength steel used for electric machinery has been disclosed in Japanese Unexamined Patent Application Publication No. 49-83613, the steel being composed of 1% to 5% of Cu, 1% to 5% of Ni, and iron as the balance. According to this technique, after solution treatment-quenching and cold rolling are repeatedly performed for steel having the above composition, aging treatment is performed, and then steel having a high strength and a low iron loss can be obtained. However, degradation in iron loss caused by aging treatment has not been satisfactorily suppressed.